Numerous sewing machines of this type are known in which the frame and the casing form a unit comprising, in its interior, in particular different support surfaces and bearings for various members of the machine, while its external surface is shaped in a manner corresponding to the "design" desired for the machine.
This type of frame-casing has been formed for a long time from metal and more recently from synthetic material, in particular plastics material, without however modification of the original concept.
It is true that adoption of plastics material has permitted a quite significant reduction in the cost of manufacture of frame-casings, but it has still not solved the major and long standing disadvantage of today's sewing machines, that is having to make use of different frames for each model of machine of different design.
A construction of sewing machine has also been proposed in which the frame and the mechanical parts that it carries are enveloped by a shell which serves only as a covering for the machine. Patent Specifications CH 257 357 and U.S. Pat. No. 3,420,200 illustrate this type of construction. The latter effectively brings improvements in the manufacture of sewing machines, since with one single type of frame it is possible to offer to customers an almost infinite number of models of different design, the different designs being dependent only on the appearance of the corresponding covering shells.